1. Field of the Invention
The present invention relates to a method of manufacturing aromatic urethane.
2. Description of the Prior Art
Various conventional methods of manufacturing aromatic urethane have been proposed. These methods are classified into a method using an aromatic nitro-compound as a starting material and a method using an aromatic primary amine as a starting material.
According to the conventional method using an aromatic nitro-compound as a starting material, an aromatic nitro-compound (e.g., nitrobenzene), an organic compound (e.g., an alcohol) containing a hydroxyl group, and carbon monoxide are allowed to react reductively with each other in the presence of a catalyst having a platinum group compound such as a palladium or rhodium compound as a major constituent to manufacture aromatic urethane. Examples of this method are described in Japanese Patent Disclosure (Kokai) Nos. 51-98240 and 54-22339 and Japanese Patent Publication No. 43-23939.
According to the conventional method using an aromatic primary amine as a starting material, aromatic primary amine (e.g., aniline), an organic compound (e.g., an alcohol) containing a hydroxyl group, and carbon monoxide are allowed to react oxidatively with each other in the presence of an oxidizer such as oxygen or an organic nitro-compound through a catalyst containing a platinum group metal compound such as a palladium or rhodium compound to prepare aromatic urethane. Examples of this method are described, e.g., in Japanese Patent Disclosure (Kokai) Nos. 55-124750, 55-120551, and 59-172451.
In either method, since use of only a platinum group metal compound as the major constituent of the catalyst results in low synthetic activity of urethane, a halogen compound such as iron chloride, iron oxychloride, vanadium oxychloride, or potassium iodide is used as an assistant catalyst. A mixture of the platinum group metal compound and the assistant catalyst are dissolved in a reacting system. However, the halogen compound greatly corrodes a metal material such as a reaction chamber and piping valves. For this reason, an expensive metal material having a good anticorrosion property must be used.
When a platinum group metal compound as a main catalyst is dissolved in a reaction solution or a solid platinum group metal compound is used, the platinum group metal partially contains a halogen compound and the halogen compound is eluted in the reaction solution. In order to recover the platinum group metal compound from the reaction solution at the end of reaction, cumbersome operations and high cost are required.
In addition, an organic compound containing a hydroxyl group for a reaction material is used as a reaction solution, and aromatic urethane has high solubility in the organic compound containing a hydroxyl group. For this reason, in order to crystallize and separate aromatic urethane from the solution after reaction, the solution must be cooled to an extremely low temperature of several tens of minus degrees in centigrade. Alternatively, the solution must be condensed and cooled to allow precipitation of crystals. Even if such precipitation is performed, it is difficult to recover aromatic urethane dissolved in the solution separately from the catalyst. Another method of recovering aromatic urethane is a distallation method. In this case, however, since the dissolved catalyst must be recovered as a distillation residue, aromatic urethane must be distilled. However, aromatic urethane is a compound having a high boiling point and must be distilled at a temperature of 100.degree. to 150.degree. C. under a high vacuum of about 1 mmHg.
Furthermore, if an aromatic nitro-compound is used as a starting material, a small amount of nonreacted aromatic nitro-compound is left in the reaction solution. If distillation is performed in this state, aromatic urethane is colored in brown by the aromatic nitro-compound.
As described above, it is difficult to separate and recover aromatic urethane from the solution and further recover the catalyst for reuse regardless of a recovery method, i.e., crystallization or distillation.
If an aromatic nitro-compound is used as a starting material an aromatic amine is by-produced, and if an aromatic primary amine is used as a starting material, N,N'-di-substituted urea is by-produced, thereby decreasing the yield of aromatic urethane.